Fizeau interferometers and the like have conventionally been employed in the measurement of surface shape through the use of interference of light. Among surface shape measurement applications, and particularly for aspheric surface shapes, so-called "null testing" is carried out. In null testing, a measurement light beam is formed using a lens, zone plate, hologram, or other such wavefront formation means for producing a wavefront corresponding to the design surface of the target surface, which may be an aspheric surface. Then, the difference between the target surface and the wavefront of the measurement light beam is measured by causing the measurement light beam, after it has been reflected at the target surface, to interfere with a reference light beam.
In performing surface shape measurements, to know the target surface shape with high precision, it has been necessary to manufacture the wavefront formation means with high precision, and to precisely control the shape of the wavefront of the measurement light beam incident on the target surface. Alternatively, it has been necessary to perform a comparative measurement of the target surface relative to a standard prototype. The standard prototype surface shape is measured beforehand with high precision, using other measurement means.
However, it is difficult to manufacture the lenses and other optical elements constituting the wavefront formation means such that radii of curvature, distances between surfaces, and so forth, of the elements are controlled with a high degree of precision. Moreover, when the target surface is aspheric, it has been difficult to obtain an aspheric prototype measured with high precision, and it is difficult to carry out surface shape measurement with high precision.